1. Field of the Invention
This invention relates to optical disc drives, and more particularly, to an apparatus and method for use on the pickup head of an optical disc drive for the purpose of adjusting the pickup head to an optimal focus point through a tree-structured search algorithm.
2. Description of Related Art
The optical disc is a mass storage medium that can hold up to several megabits or even gigabits of binary data in a single piece of plastic disc, which is several times larger than a conventional magnetic disk can hold. Due to its high capacity, the optical disc is now widely used in the computer arena as the major data storage medium.
An optical disc drive is composed of a large number of constituent parts including motors and optical and electronic components. In operation, the characteristics of these constituent parts can change in time and be affected by various environmental conditions. The changes in the characteristics can then affect the gain of the servo control. loop for focusing/tracking control of the pickup head of the optical disc drive. Moreover, the gain can be further affected by changes in the optical properties of the photo sensors and the reflectivity of the optical disc being used. Therefore, the gain of the servo control loop should be always adaptively adjusted to compensate for any changes in the system characteristics before being put into actual operation so as to allow the optical disc drive to operate without errors in focusing and tracking.
In conventional optical pickup heads, a laser diode is used to generate a laser beam which is directed at the data area on the optical disc. Then, the reflections of the laser beam from the surface of the optical disc are detected by a specially designed photo sensor that can generate opto-electrical signals indicative of the focusing/tracking errors in the pickup head. The photo sensor includes a number of separate light-sensitive areas, typically six, with four of them being used to generate the focusing error (FE) signal and two of them being used to generate the tracking error (TE) signal. When illuminated by the reflections from the optical disc, these six light-sensitive areas will each produce an opto-electrical signal whose amplitude is proportional to the intensity of the light spotted thereon. Through current-to-voltage conversion means and preamplifiers, these opto-electrical signals can be transformed into the FE signal, the TE signal and the RF (radio frequency) signal. The FE/TE signals can then be used in a servo feedback control means to adjust the pickup head to the correct focusing and tracking positions.
FIG. 1 is a graph used to depict the relationship between focus bias and the amplitude of the resultant RF signal. In operation, it is desired that the focus bias be made as close to zero as possible. As shown in FIG. 1, the amplitude of the RF signal is at the maximum amplitude (point a) when the focus bias is exactly zero, and decreases with the increase in the focus bias. Ideally, the FE signal should be well balanced, just like the waveform illustrated in FIG. 2A. However, due to some inconsistency in the reflectivity of the photo sensors and the gain of the pre-amplifiers, the FE signal can be unbalanced as illustrated by the waveform in FIG. 2B.
In operation, it is desirable to position the pickup head at the focus point where the focus bias is exactly or very close to zero, which allows the amplitude of the RF signal to be at the maximum. Two conventional methods have been proposed for this purpose. The first method is through manual adjustment in manufacture. This method, however, is quite laborious and time-consuming, and thus very cost-ineffective.
The second method is disclosed in the U.S. Pat. No. 5,610,886, "FOCUS BALANCE AUTOMATIC ADJUSTING DEVICE AND AUTOMATIC ADJUSTING METHOD" to Hayashi et al.
Assume the pickup head is initially located at the point b (which is deviated from the optimal focus point a) due to the unbalanced condition of the FE signal as illustrated in FIG. 2B. This patent first uses the point b as a center point and defines a pair of focus bias points respectively on both sides of the center point, then compares the amplitudes of the RFRP signals resulted from these two focus bias points with the maximum amplitude at the optimal focus point a. The focus bias point that has the minimum difference is subsequently used as the center point for the next search. This procedure is repeated until the pickup head is substantially positioned at the optimal focus point a.
One drawback to the foregoing patent, however, is that if the initial focus bias point is very far away from the optimal focus point a, for example at the point c in FIG. 1, the procedure for finding the optimal focus point a becomes very time-consuming, causing the access time to be lengthy.
As a summary, the conventional manual adjustment method is quite laborious and time-consuming to carry out, which causes the manufacture to be highly cost-ineffective. Although the Hayashi patent can perform the adjustment automatically and fast, the access time can be lengthy and that will cause the read operation to be slow.